Current mirrors, which are used primarily as a means for replicating a reference current, are employed in a variety of analog circuits, such as, but not limited to, reference generators, amplifiers, and digital-to-analog converters. A current mirror is designed to receive a reference current and to generate an output current which is identical to, or proportional to, the reference current at an output of the current mirror. A current mirror typically includes a current source and/or a current sink, and a bias circuit for biasing the current source and/or current sink. An ideal current mirror may be characterized as having essentially an infinite parallel output impedance, such that its output current is independent of the voltage at its output. The ideal current mirror is also independent of semiconductor process, supply voltage, and/or temperature (PVT) variations. In practice, however, the output impedance of a current mirror is finite, such that the output current generated by the current mirror is influenced, at least to some extent, by variations in the voltage at the output of the current mirror. Moreover, both the output impedance and the output current generated by the current mirror are typically affected by PVT variations to which the current mirror may be subjected.
Cascode current mirrors represent a particular class of current mirrors which typically include two or more transistor devices connected in a stacked configuration, thereby significantly increasing an output impedance of the current mirror. Such cascode current mirrors are designed to replicate the reference current with a higher degree of precision compared to simple current mirrors (e.g., Wilson current mirror, etc.), and are therefore preferred. Unfortunately, however, cascode current mirrors typically require more voltage headroom to operate properly, and therefore cannot tolerate as large of an output voltage swing as can be tolerated by simple current mirror arrangements. Voltage headroom may be characterized as the output voltage of the current mirror below which one or more transistor devices in the current mirror have gone out of a saturation region of operation. Consequently, most cascode current mirrors are not suitable for use in a low-voltage power supply environment.
While several improvements have been made to the basic current mirror configuration, these improvements still have one or more disadvantages associated therewith. These disadvantages include, but are not limited to, low output impedance, low output voltage swing, high susceptibility to PVT variations, etc. Accordingly, there exists a need for an improved bias circuit for biasing a cascode current source and/or current sink that does not suffer from one or more of the problems exhibited by conventional bias circuits.